topography topics

Managing Pre-Op
Refractive Surgery Patients

BY KENNETH A. LEBOW, OD
June 2000

Corneal topography can help the practitioner and patient understand when to schedule refractive surgery. Operating on eyes before they have
stabilized increases the risk for post-operative
enhancements. While postponing surgery increases the likelihood that the patient may forgo the surgery, performing serial topography along with comparative refractive analysis demonstrates the importance of waiting.

The key concerns are what years of contact lens wear have done to the cornea, and how long it will take to achieve refractive stability once lenses are removed. As a general rule, RGP lens wearers should remove their lenses for approximately one month for every decade in contact lens wear.

Difference Maps

Using the Difference Display to track these changes is the easiest way to visualize progress. Figure 1 compares a normal cornea sphericalized from years of RGP lens wear to the same eye after three months of nonwear. While a slight with-the-rule astigmatism (-1.62cx006 FM 43.37) is visible after contact lens wear, it is reduced compared to non-wearing levels (-2.50cx008 FM 44.25). The cornea steepened by over 2.50D inferiorly after lens removal. The contact lens-induced flattening effect can be easily seen from the large area of steepening visible from the Difference Map. Until repetitive maps show no significant difference, surgery should not be scheduled. Figure 2 shows the stability achieved after discontinuing lens wear for nine weeks. The Difference Map (bottom) is uniform in color and mostly green, suggesting no significant difference between the two topography maps.

Statistical indices show a significant change in the Shape Factor between contact lens and post contact lens wear. It increased from 0.08 to 0.24, indicating that the cornea became more prolate after lens removal. Further, it shows that the superior flattening and inferior steepening was induced by contact lens wear (Figure 1 upper left map), and is not keratoconus. The CIM remained virtually unchanged (0.83 to 0.74), demonstrating no corneal distortion was present. The TKM (44.8D to 46.2D) increased significantly, indicating that the post-lens wearing cornea was sphericalized, and removing the lens allowed the astigmatism to return to pre-fitting levels. The Reference Sphere (40.4D to 41.1D) also increased slightly, showing a steepening corneal curvature. These are all normal corneal changes associated with the discontinuation of contact lens wear.